The Best Way to Prevent Your Windshield From Fogging Up
When you drive in winter, you have to deal with a lot of foggy windshields. What’s the best way to clear this fog? Some people will tell you to turn off the heating, others will tell you to turn on the air conditioning. Who’s right? And when should you just hit the defrost button instead? I’ll explain.
To prevent fogging in cold weather, turn on the heat and air conditioning.
I’ll get to the more detailed scientific explanation in a minute, but the short answer is this: To fog up your windshield quickly in cold weather, you need to turn on the heat and air conditioning.
Yes, you can do both at the same time. If you have a dial that controls the temperature, set it to the highest temperature. Then look around for the button that turns the air conditioner on or off. Click on it to turn it on.
Some cars have a button that does everything for you. My car has a heated windshield button that does the following:
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Directs the fan to the windshield
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Turns the fan on at full power and increases the temperature.
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Includes air conditioning
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Disables recirculation
This is exactly what you need to do if you operate the controls yourself: turn on the heat, turn on the air conditioning, and make sure the air is blowing full force onto the windshield (look for the windshield icon instead of the icon with arrows pointing at a person). Turning off recirculation helps because you will be drawing in dry air from outside instead of recirculating slightly moist air from inside.
How to protect your windshield from fogging in hot weather
If you’re reading this on a hot summer day, just turn on your wipers. The fog is on the outside of the windshield, so you can simply wipe it off.
What is the difference between defrosting and defogging?
Okay, some definitions. Fog consists of tiny droplets of water that condense on the windshield. You will find this whenever you have a temperature difference between two sides of the glass. Fog forms on the warmer side (inside the car in winter).
Frost , on the other hand, is a layer of ice. Most often you will see it on the outside of the car and it is what you scrape off with an ice scraper. In very cold weather, the fog on the inside of the windshield can freeze, forming frost on the inside. Treat it the same way you treat fog.
Your car’s rear defrost button heats a set of wires embedded in the glass. They melt the ice on the rear window.
Your car’s front defrost button defrosts the windshield by warming it and prevents fogging by drying the air to evaporate fog. It is both a heater and a heater.
Consult your vehicle’s owner’s manual to find out what settings your vehicle has and how they work. The rear defrost button is very common even in older cars; The front defrost button is a newer development, but is still widely available.
The Science of Anti-Fog
There are two things going on here: in the short term (seconds to minutes) and in the long term (the rest of your trip).
Warmth prevents fogging (and frost)
In the long run, you can prevent fogging by ensuring both sides of the windshield are at the same temperature. Fog builds up on your windshield for the same reasons that a cold drink “sweats” on a hot day. When one side of the glass is significantly cooler than the other, moisture from the warm air will condense on the warmer side of the glass (since the glass is cooler compared to the warm air).
This means that in winter, condensation from warm indoor air will fall onto the still cold windshield. If you can get your windshield warm enough, it will stop pulling moisture out of the air.
In summer it’s the other way around. If you’ve cooled the inside of your car and it’s 100 degrees outside, you may get moisture from the outside air condensing on the windshield. However, this is not a heater situation: the condensation is outside, so you simply wipe it off with your wipers.
The air conditioner dries up the already accumulated fog
So why isn’t heat alone enough to do the job? Well, first of all, sometimes it’s too cold for the windshield to be warm enough to prevent fogging. But more importantly, you want to fog up your windshield, not just prevent fogging in the future. That’s why you turn on the air conditioner.
Air conditioners don’t just cool the air; they dry it out. When you turn on the heat and air conditioning at the same time, you get hot, dry air. This leads to heating of the windshield and at the same time drying out the moisture. Turn the air conditioning and heat on full blast and often the entire windshield is cleared in a matter of seconds.
If you’ve heard the advice to open your windows, this may help for the same reason: the flow of cool, dry air will help dry out the fog. But an air conditioner usually does the same job faster without cooling the entire car. So I would stick with air conditioning.
How to prevent car windows from fogging up
Since windshield fogging is caused by moisture inside the car, keeping the car’s interior dry will help. If you’ve gotten a lot of snow and slush into your car, clear it out.
You can also start warming up your windshield before driving. If you have a remote start, great! But even if not, the first step in preparing your car for winter driving should be to start it and warm it up. Heat your windshield while you clean the outside.
Lastly, consider cleaning the inside of your windshield because clean glass does not fog up as easily. No, not with your hands or those fast food napkins you found under your seat; sebum, various fats and dirt will only aggravate the problem. Use a suitable glass cleaner such as Windex.